To avoid having to answer any more questions on the new features the Athlon 64/Opteron and Intel Pentium 4 Prescot, I’ve made this little FAQ to help answer any questions anyone might have.
For the crib sheet to that, 64 bit Integer calculations are one of the things that make 64 bit CPUs so great – they can make 64 bit calculations in hardware rather than emulating it with a bunch of 32 bit calculations. The address space is the logical address space the CPU can handle. The AMD64 and EM64T CPUs do not have a full 64 bits of address space, as they won’t need it for a while. They use 64 bit addresses and truncate them internally. PSE36 and PAE36 are technologies that allow programs to use 36 bits (64 Gigabytes) of memory on a 32 bit CPU. It’s been used by Intel for years now to extend the memory capability of its Xeon line, although it’s kind of ugly. PSE36 is the technology that allows for 36 bits of virtual address space and PAE36 for 36 bits of physical address space. A program may use one or both of these. The 40 bit physical memory addressing means a maximum of 1 TB physical memory, coupled with 256 TB of logical address space (this includes virtual memory and allows for tons of large, contiguous blocks of unused address space). Remember, a logical address does not necessarily need to match up with the same physical memory address. The automatic clock control will be of overclocking interest, as it means the CPU will have a few unlocked multipliers. By purchasing a 200(800) FSB Pentium 4 with this technology, one could turn down the multiplier and turn up the CPU speed by increasing the bus to 266(1066) FSB while still having all expansion busses running in sync.
Of course, you may be wondering about the availability of said processors. All of them except the Prescott E0 are pretty available, and Prescott E0 availability should be ramping quickly within a few weeks of this writing, at which point hopefully the press hype will make this FAQ obsolete.
There are a few true 64 bit CPUs around, but for all intents and purposes the Intel and AMD CPUs are both 64 bit. However, the arguement stems from their different approaches to this. K8 extends 32 bit to 64 and allows use of more registers and more computation units, making it perform better in 64 bit than 32. Prescott extnds its existing 32 bit capability to 64 bit in a more traditional manner, making 64 bit calculations run modestly slower than 32 bit. Traditionally, all CPUs when going to 64 bit have used this method. The third is used by the Itanium and Itanium only, which is a CPU that only has a 64 bit mode and must emulate 32 bit processing.
So now that we’ve gotten that out of the way, using the common definition you may ask what 64 bit CPUs are available. There’s a lenghty list:
And there are many others, but those are the ones you’ll run across the most.
If you have any more questions, ask them in this thread that way we can keep all the 64 bit information together for easier consumption.
Edited to clarify the available address space (I looked it up for a difinitive answer)